serial: fsl_linflexuart: deduplicate character sending

Introduce a new linflex_put_char() helper to send a character. And use
it on both places this code was duplicated.

Cc: Stefan-gabriel Mirea <stefan-gabriel.mirea@nxp.com>
Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Link: https://lore.kernel.org/r/20220124071430.14907-9-jslaby@suse.cz


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>